Type Anytime Upgrade in the Search programs and files box in the Start menu and click on the Windows Anytime Upgrade icon. From there, you can buy an Anytime Upgrade to Windows 7 Ultimate. Then your can enter your Anytime Upgrade product key and perform a simple upgrade to Windows 7 Ultimate so you can install a different language. All programs and files will stay on the computer.

https://answers.microsoft.com/en-us/windows/forum/windows_7-windows_install/upgrade-windows-7-home-premium-in-german-language/e246b709-fd5e-e011-8dfc-68b599b31bf5